By performing a catalytic gas-phase oxidation of at least one or more acrylic acid raw material selected from the group consisting of propane, propylene and acrolein (hereinafter, referred to as “propylene etc.”) by a molecular oxygen-containing gas in the presence of oxidation catalyst, in addition to acrylic acid as the object, by-products such as acetic acid, formaldehyde, acrolein, propionic acid, maleic acid, acetone, furfural, benzaldehyde, protoanemonin, and mixed gas containing impurities are obtained as a reaction product. Next, the purified acrylic acid can be obtained by purifying this reaction product by various purification methods to remove these by-products and impurities.
For example, in a method for producing acrylic acid industrially by the catalytic gas-phase oxidation of propylene etc. by molecular oxygen-containing gas, there is a method that an aqueous solution containing acrylic acid and by-product such as acetic acid is obtained by a step that mixed gas obtained by performing the catalytic gas-phase oxidation is introduced into a collection column, where contacting with an aqueous solution for collection such as water, cooling, and absorbing for collection are carried out, then, acrylic acid is separated from this aqueous solution by distillation or the like, and is purified to obtain the purified acrylic acid.
Further specifically, as a method to obtain the high-purity acrylic acid as a product from the above aqueous solution containing acrylic acid, there are a method that, in azeotropic separation column, the high-purity acrylic acid substantially not containing acetic acid, water and poor water-soluble solvent can be recovered from the column bottom by distilling using poor water-soluble solvent (for example, refer to: JP-A-5-246941, EP-0551111-A1); a method that the purified acrylic acid having high-purity can be obtained by a comparatively easy step by introducing an acrylic acid-containing solution obtained from the collection step to a crystallization step (for example, refer to: JP-A-2005-15478, US-2004249199 A1), or the like.
However, in the purified acrylic acid obtained from the above method, there still exist minor amount of impurities which is difficult to be completely removed by conventional distillation and/or crystallization. These impurities are known to have a drawback that induction time in polymerization reaction is forced to lengthen when acrylic acid is homo-polymerized or copolymerized with the monomer co-polymerizable with this monomer, or they perform an action of chain-transfer agent resulting in the formation of polymer having low polymerization degree (for example, refer to JP-A-61-218556). Various measures that for aiming to prevent the polymerization delay caused by among the various impurities, particularly, protoanemonin, this protoanemonin in the purified acrylic acid is removed, or the concentration of protoanemonin is reduced, are provided. It should be noted that, protoanemonin is known to have an effect of polymerization inhibition for the acrylic acid-containing solution (for example, refer to: JP-A-47-17714).
As a method for treating the above protoanemonin, specifically, following methods are provided.
(1) a method that the impurities (protoanemonin) contained in the crude acrylic acid obtained by catalytic gas-phase oxidation of propylene or acrolein are treated with aqueous solution of bisulfite in the purification step thereof, and further, are treated with hydrazine compounds (for example, refer to: JP-A-61-218556);
(2) a method that impurities (protoanemonin) contained in the crude methacrylic acid obtained by catalytic gas-phase oxidation of isobutylene or the like is treated with the aqueous solution of bisulfite in the purification step thereof (for example, refer to: JP-A-59-93027);
(3) a method that an effective amount of p-phenylene diamine and the salt thereof are added to aqueous solution of methacrylic acid (for example, refer to JP-3359368);
(4) a method that super purified acrylic acid is obtained further by reducing protoanemonin from the purified acrylic acid with performing additional purification treatment such as increase of theoretical plate number, increase of reflux ratio in distillation column, and increase of crystallization times, or super purified acrylic acid is obtained further by strong alkaline treatment of this product (for example, refer to: WO 01/098382).